Identification method of an OFDM signal

ABSTRACT

An OFDM signal identification method in a received signal, wherein the received signal is sampled and cut off into blocks of P samples, each block being subjected to an inverse Fourier transform to obtain a plurality of symbols at a plurality of frequencies. For each frequency of the plurality, the kurtosis of the set of symbols thus obtained at this frequency is calculated, and then it is determined whether the kurtosis has a periodicity in frequency domain from kurtosis values calculated for the frequencies of the plurality, and the presence of an OFDM signal in the received signal is identified if the kurtosis has such a periodicity.

TECHNICAL FIELD

The present invention relates to a blind identification method of anOFDM signal. This method is in particular applicable to so-calledopportunistic telecommunication systems in the field of cognitive radio.

STATE OF PRIOR ART

An opportunistic telecommunication should permanently know its spectralenvironment in order to adapt the transmission parameters thereof inorder not to interfere with existing telecommunication systems, calledprimary telecommunication systems.

Most primary telecommunication systems with which an opportunistic useof the spectrum is contemplated are OFDM telecommunication systems.Accordingly, it matters for an opportunistic terminal whether an OFDMsignal is present in its environment and, if any, to determine theparameters of this signal.

The OFDM modulation is well-known in the state of the art and isutilized in numerous telecommunication systems such as DVB-T, ADSL,Wi-Fi (IEEE 802 a/g), WiMax (IEEE 802.16). It has the advantage of agood spectral efficiency and a good protection against frequencyselective fadings.

It is reminded that in an OFDM system, the information symbols to betransmitted are grouped by blocks of N symbols, where N is generally apower of 2, the OFDM symbols being obtained by performing an IFFT(Inverse Fast Fourier Transform) on said blocks of information symbols.Generally, a cyclic prefix is added at the beginning of each OFDM symbolto absorb the inter-symbol interference or ISI and make equalizationeasier upon reception. The OFDM signal made of these OFDM symbols canthen be possibly translated into frequency.

Generally, the signal emitted by an OFDM system can be represented inbaseband by:

$\begin{matrix}{{s_{a}(t)} = {\frac{\sqrt{E}}{N}{\sum\limits_{k}{{g\left( {t - {kT}_{s}} \right)} \cdot {\sum\limits_{n = 0}^{N - 1}{a_{k,n}{\mathbb{e}}^{{- 2}\;{\mathbb{i}}\;\pi\frac{n}{{NT}_{c}}{({t - {DT}_{c} - {kT}_{s}})}}}}}}}} & (1)\end{matrix}$where T_(S)=(N+D)T_(C) is the duration of an OFDM symbol, E is the powerof the signal, N is the number of carriers of the OFDM multiplex,a_(k,n) are the information symbols relating to block k, belonging to aM-ary modulation alphabet, typically BPSK, QPSK or QAM, 1/T_(c) is theinformation symbol rate where T_(c) is the “chip” time, D is the size ofthe cyclic prefix expressed in number of chips, g(t) is OFDM symbolsshaping pulse having a time frame [0,(N+D)T_(c)] for apodizing thespectrum of the signal.

An OFDM signal is schematically represented in FIG. 1. It is made of asequence of OFDM symbols, each symbol having a total duration (N+D)T_(c)of which a useful duration NT_(c) and a guard interval of durationT_(prefix)=DT_(c), wherein the cyclic prefix is located. It is remindedthat conventionally, the cyclic prefix is a copy of the end of the OFDMsymbol within the guard interval. In some OFDM systems, cyclic prefixesare merely omitted, in other words the useful parts of symbols areseparated by “empty” guard intervals.

The known methods for identifying an OFDM signal make use for the mostpart of the statistical properties of the signal caused by the presenceof the cyclic prefix. However, these methods turn out to be poorlyefficient when the length of the cyclic prefix is low with respect tothe useful duration of the ODFM symbol, which is very common inpractice, in particular in WiMax and DVB-T systems. Furthermore, thesemethods yield poor results when the transmission channels are of themulti-path type.

An identification method using the time periodicity of theauto-correlation function of the OFDM signal was described in the paperof A. Bouzegzi et al. entitled “A second order statistics basedalgorithm for blind recognition of OFDM based systems” published in IEEEGlobal Telecommunications Conference (Globecom), November 2008. If thismethod has a high robustness for small sizes of the cyclic prefix andfor multi-path channels, it however requires the presence of a cyclicprefix in OFDM symbols.

Several identification methods have been recently provided to discardthe cyclic prefix, in particular the one described in the paper of A.Bouzegzi et al. entitled “A fourth-order based algorithm forcharacterization of OFDM signals” published in IEEE Workshop on SignalProcessing Advances in Wireless Communications (SPAWC), July 2008. Thismethod, based on an entropy minimization criterion at the output of anOFDM demodulator, requires a time and frequency synchronization which ishardly compatible with a real time implementation.

Therefore, the object of the present invention is to provide a blindidentification method of an OFDM signal which does not depend on thepresence and the nature of the prefix used, in particular of its cycliccharacter, but without requiring a time and/or frequencysynchronization.

Another object of the present invention is to provide an OFDM signalidentification method independent of the length of the prefix used,whether cyclic or not, which is robust to both noise and multi-paths.

DESCRIPTION OF THE INVENTION

The present invention is defined by an OFDM signal identification methodin a received signal, wherein the received signal is sampled and cut offinto blocks of P samples, each block (B_(l)) being then subjected to aFourier transform to obtain a plurality of symbols l (â_(l)(f)) at aplurality of frequencies. For each frequency of said plurality, iscalculated the kurtosis (κ(f)) of the set of symbols (â_(l)(f), l=1, . .. , L) thus obtained at this frequency, and then it is determinedwhether said kurtosis has a periodicity in the frequency domain fromkurtosis values calculated for the frequencies of said plurality, andthe presence of an OFDM signal is identified in the received signal ifthe kurtosis has such a periodicity.

Advantageously, to determine whether said kurtosis has a periodicity inthe frequency domain, its spectrum is calculated and it is determinedwhether it has a spectrum peak for a value having a period (Δf).

Then, the average value ( κ) of the kurtosis can be extracted therefromand the inverse Fourier Transform of the kurtosis thus centred iscalculated, a spectrum peak being detected if its height (α_(P)) ishigher than a predetermined threshold (α_(T).).

According to another advantageous embodiment, it is attempted toidentify the presence of an OFDM signal having a given OFDM symbolduration T_(s). The number of samples per block is then selected suchthat

${P_{opt} \approx 1},{5\frac{T_{s}}{T_{e}}}$where T_(e) is the sampling period.

According to one alternative, the cutting off of the signal sampled intoblocks of P samples is repeated for a plurality of values of P, thekurtosis calculation is repeated for said plurality of values of P, thekurtosis spectra thus obtained are calculated, the spectrum peak height(α_(P)) is determined in each of these spectra (H_(P)) and the valueP=P_(opt) corresponding to the highest peak height is selected.

In this case, for the selected P_(opt) value, it is advantageouslydetermined whether the spectrum peak has a height (α_(P opt)) higherthan a predetermined threshold value (α_(T)).

The subcarrier spacing of said OFDM signal can also be determined fromthe position of the spectrum in the kurtosis spectrum obtained for theselected P_(opt) value.

Finally, the useful duration of an OFDM symbol can be determined in saidOFDM signal from the position of the spectrum peak in the kurtosisspectrum obtained for the selected P_(opt) value.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will becomeclearer upon reading preferable embodiments of the invention, inrelation with the appended figures wherein:

FIG. 1 schematically illustrates an OFDM signal;

FIG. 2 schematically illustrates a sampling window of an OFDM signal;

FIG. 3 schematically represents a flow chart of the OFDM signalidentification method according to an embodiment of the invention;

FIG. 4A illustrates the variation of the kurtosis of a group of symbolsassessed by frequency, for an OFDM signal being propagated in a BBAGchannel;

FIG. 4B shows the spectrum of the kurtosis of FIG. 4A;

FIG. 5A illustrates variations of the kurtosis of a group of symbolsassessed by frequency, for an OFDM signal being propagated in amulti-path type channel;

FIG. 5B shows the spectrum of the kurtosis of FIG. 5A;

FIG. 6 represents one exemplary variation in the peak height of thekurtosis spectrum as a function of the ratio of the sampling windowwidth to the duration of an OFDM symbol.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

An opportunistic terminal attempting to identify an OFDM signal will beconsidered in the following, that is attempting to determine whether anOFDM signal is present in its environment and, if necessary, to obtainat least one modulation parameter for this signal. By modulationparameter of the OFDM signal, it is meant here the useful duration ofthe OFDM symbol (T_(u)=NT_(c)) or equivalently, the subcarrier spacing(1/NT_(c)), the prefix duration (DT_(c)), the total duration of the OFDMsymbol (T_(s)=(N+D)T_(c)).

It will be supposed that the OFDM signal has the form given by theexpression (1), if present.

The opportunistic terminal performs a sampling of the signal received inbaseband, the carrier frequency being assumed to be known or belongingto a set of known discrete values. If f_(e)=1/T_(e) is the samplingfrequency used, chosen sufficiently high to meet the Nyquist criterionrelating to the bandwidth of a possible OFDM signal. Thus sampled signalis cut off into L blocks of P samples. The samples of block l can beexpressed as:s _(l,a)(p)=s _(a)(pT _(e) −lPT _(e))Π_(PT) _(e) (pT _(e) −lPT_(e))  (2)where Π_(PT) _(e) is the window function having the width PT_(e) suchthat Π_(PT) _(e) (u)=1 if 0≦u<PT_(e) and Π_(PT) _(e) (u)=0, otherwise, lis the block index, noted B_(l). The time width PT_(e) of a block ischosen sufficiently high to be higher than commonly used durations ofthe OFDM symbol.

FIG. 2 schematically shows the relative positions of the blocks and ofthe OFDM symbols of the signal received.

For a given block B_(l), T₀ stands for the time separating the beginningof the block and the beginning of the first OFDM symbol contained inthat block. Similarly, T₁ stands for time separating the end of theblock and the end of the last OFDM symbol contained in that block. Indexl has been omitted from T₀ and T₁ to simplify notation. It will beunderstood that as a general rule, blocks B_(l) are not synchronizedwith OFDM symbols.

Once the signal has been sampled and cut off in blocks, a FourierTransform of each block B_(l), typically an FFT, is performed to assessthe information symbol carried by the carrier f of this block, that is:

$\begin{matrix}{{{\hat{a}}_{l}(f)} = {\sum\limits_{p = {l\; P}}^{{{({l + 1})}P} - 1}{{S_{a}\left( {pT}_{e} \right)}{\mathbb{e}}^{2\;{\mathbb{i}}\;\pi\;{pT}_{e}f}}}} & (3)\end{matrix}$

Thus, a sequence of symbols assessed is obtained for each frequencyâ_(l)(f), l=1, . . . , L where L is the considered number of consecutiveblocks.

It will be assumed here that the information symbols a_(k,n) can berepresented by independent and equally distributed random variables.More precisely, these random variables are therefore independent from aOFDM symbol to another and from a carrier to another. The OFDM symbolscan contain a prefix or not, whether cyclic or not.

In order to show the independence and robustness of the identificationmethod provided, it will be assumed that the prefix length is zero.

Finally, a noise-free Gaussian channel will be first considered, and thecase of a real channel will be then contemplated.

The identification method according to the invention is based on thefollowing remarkable property:

∀fε[0,1/T_(c)], if f+Δfε[0,1/T_(c)] we have:

$\begin{matrix}{{\forall l},{{\kappa\left( {{\hat{a}}_{l}(f)} \right)} = {{\kappa\left( {{\hat{a}}_{l}\left( {f + {\Delta\; f}} \right)} \right)} + {O\left( \frac{1}{N} \right)}}}} & (4)\end{matrix}$where κ(â_(l)(f)) is the kurtosis of â_(l)(f) considered as a randomvariable. In other words, for a given frequency f, the kurtosis ofsymbols assessed on this frequency is the quasi periodic function in aquasi-period Δf.

The property (4) can be demonstrated as follows:

$\begin{matrix}\begin{matrix}{{{\hat{a}}_{l}(f)} = {\sum\limits_{p = {l\; P}}^{{{({l + 1})}P} - 1}{{S_{a}\left( {pT}_{e} \right)}{\mathbb{e}}^{2\;{\mathbb{i}}\;\pi\;{pT}_{e}f}}}} \\{= {\sum\limits_{k}{\frac{1}{N}{\sum\limits_{n = 0}^{N - 1}{a_{k,n}{\mathbb{e}}^{{- 2}{\mathbb{i}}\;\pi\; n\;\Delta\;{fkT}_{s}}}}}}} \\{\sum\limits_{p = {l\; P}}^{{{({l + 1})}P} - 1}{{S_{a}\left( {pT}_{e} \right)}{\mathbb{e}}^{{- 2}{\mathbb{i}}\;\pi\;{{pT}_{e}{({{n\;\Delta\; f} - f})}}}{g\left( {{pT}_{e} - {kT}_{s}} \right)}}}\end{matrix} & (5)\end{matrix}$

Since the block B_(l) is not synchronous with the OFDM symbols, it willgenerally comprise a first part made of an OFDM symbol end portionhaving an index k₀, a second part made of an integer K≧1 of OFDM symbolshaving respective indexes k₁, . . . , k_(K) and a third part made of anOFDM symbol beginning portion having the index k_(K+1).

By performing this decomposition in three parts and by introducing thenotation ã_(k,n)=a_(k,n)e^(−2iπnΔfkT) ^(s) , the expression (5) becomes:

$\begin{matrix}{{{\hat{a}}_{l}(f)} = {\frac{1}{N}{\sum\limits_{n = 0}^{N - 1}\left\lbrack {{{\overset{\sim}{a}}_{k_{0},n}{\sum\limits_{p = {\ell\; P}}^{{l\; P} + {\lfloor{T_{0}/T_{e}}\rfloor}}{\mathbb{e}}^{{- 2}{\mathbb{i}}\;\pi\;{{pT}_{e}{({{n\;\Delta\; f} - f})}}}}} + {\sum\limits_{k = k_{1}}^{k_{K}}{{\overset{\sim}{a}}_{k,n}{\sum\limits_{p = 0}^{{\lfloor{T_{s}/T_{e}}\rfloor} - 1}{\mathbb{e}}^{{- 2}{\mathbb{i}\pi}\;{{pT}_{e}{({{n\;\Delta\; f} - f})}}}}}} + {{\overset{\sim}{a}}_{k_{K + 1},n}{\sum\limits_{p = {{{({l + 1})}P} - {\lfloor{T_{1}/T_{e}}\rfloor}}}^{{{({l + 1})}P} - 1}{\mathbb{e}}^{{- 2}{\mathbb{i}}\;\pi\;{{pT}_{e}{({{n\;\Delta\; f} - f})}}}}}} \right\rbrack}}} & (6)\end{matrix}$where └A┘ is the highest integer equal to or lower than x, that is else:

$\begin{matrix}{{{{\hat{a}}_{l}(f)} = {\frac{1}{N}{\sum\limits_{n = 0}^{N - 1}\left\lbrack {\sum\limits_{k = {k\; 0}}^{k_{K + 1}}{{\overset{\sim}{a}}_{k,n}{\mathbb{e}}^{{\mathbb{i}}\;\theta_{k}}{\Lambda_{k,n}(f)}}} \right\rbrack}}}{with}\text{:}} & (7) \\{{{{\Lambda_{k,n}(f)} = \frac{\sin\left( {\pi\;{T_{s}\left( {{n\;\Delta\; f} - f} \right)}} \right.}{\sin\left( {\pi\;{T_{e}\left( {{n\;\Delta\; f} - f} \right)}} \right.}},{k = k_{1}},\ldots\mspace{14mu},k_{K}}{{\Lambda_{k_{0},n}(f)} = {\frac{\sin\left( {\pi\;{T_{0}\left( {{n\;\Delta\; f} - f} \right)}} \right.}{\sin\left( {\pi\;{T_{e}\left( {{n\;\Delta\; f} - f} \right)}} \right.}\mspace{14mu}{and}}}{{\Lambda_{k_{K + 1},n}(f)} = \frac{\sin\left( {\pi\;{T_{1}\left( {{n\;\Delta\; f} - f} \right)}} \right.}{\sin\;\left( {\pi\;{T_{e}\left( {{n\;\Delta\; f} - f} \right)}} \right.}}} & (8)\end{matrix}$and where e^(tθ) ^(k) is a phase term.

The kurtosis of the assessed symbols is by definition given by:

$\begin{matrix}{{\kappa\left( {{\hat{a}}_{l}(f)} \right)} = \frac{{cum}_{4}\left( {{\hat{a}}_{l}(f)} \right)}{\left\{ {E\left( {{{\hat{a}}_{l}(f)}}^{2} \right)} \right\}^{2}}} & (9)\end{matrix}$where cum₄(â_(l)(f)) is the 4-order moment (4-order cumulant) ofâ_(l)(f) and E(|â_(l)(f)|²) is the variance of â_(l)(f) (having a zeroaverage).

The numerator of the expression (9) can also be written as:

$\begin{matrix}{{{cum}_{4}\left( {{\hat{a}}_{l}(f)} \right)} = {{{cum}_{4}\left( {\frac{1}{N}{\sum\limits_{n = 0}^{N - 1}\left\lbrack {\sum\limits_{k = k_{0}}^{k_{K + 1}}{{\overset{\sim}{a}}_{k,n}{\mathbb{e}}^{{\mathbb{i}}\;\theta_{k}}{\Lambda_{k,n}(f)}}} \right\rbrack}} \right)} = {\frac{1}{N^{4}}{\sum\limits_{n^{(1)},n^{(2)},n^{(3)},n^{(4)}}{\quad\left( \begin{matrix}{{{\overset{\sim}{\Lambda}}_{k^{(1)},n^{(1)}}(f)}{{\overset{\sim}{\Lambda}}_{k^{(2)},n^{(2)}}^{*}(f)}{{\overset{\sim}{\Lambda}}_{k^{(3)},n^{(3)}}(f)}{{\overset{\sim}{\Lambda}}_{k^{(4)},n^{(4)}}^{*}(f)}} \\{\sum\limits_{k^{(1)},k^{(2)},k^{(3)},k^{(4)}}{E\begin{pmatrix}{{{\overset{\sim}{a}}_{k^{(1)},n^{(1)}}{\overset{\sim}{a}}_{k^{(2)},n^{(2)}}^{*}},} \\{{\overset{\sim}{a}}_{k^{(3)},n^{(3)}},{\overset{\sim}{a}}_{k^{(4)},n^{(4)}}^{*}}\end{pmatrix}}}\end{matrix} \right)}}}}} & (10)\end{matrix}$where n⁽¹⁾ and k⁽¹⁾ have been distinguished for convenience purposes,i=1, . . . , 4 the four indexes n and k being involved in thecalculation of the moment and where {tilde over (Λ)}_(k,n)(f)=^(etθ)^(k) Λ_(k,n)(f).

Since the information symbols are random variables independent from asymbol to another and from a subcarrier to another, all the crossedterms of the expression (10) are zero and the 4-order cumulant issimplified as:

$\begin{matrix}{{{cum}_{4}\left( {{\hat{a}}_{l}(f)} \right)} = {{{cum}_{4}\left( {{\overset{\sim}{a}}_{l}(f)} \right)}\left( {\frac{1}{N^{4}}{\sum\limits_{n = 0}^{N - 1}{\sum\limits_{k = k_{0}}^{k_{K + 1}}{{\Lambda_{k,n}(f)}}^{4}}}} \right)}} & (11)\end{matrix}$

Similarly, the denominator of the expression (9) can be written as:

$\begin{matrix}{{E\left( {{{\hat{a}}_{l}(f)}}^{2} \right)} = {{E\left( {\overset{\sim}{a}}^{2} \right)}\left( {\frac{1}{N^{2}}{\sum\limits_{n = 0}^{N - 1}{\sum\limits_{k = k_{0}}^{k_{K + 1}}{{\Lambda_{k,n}(f)}}^{2}}}} \right)}} & (12)\end{matrix}$

From the expressions (11) and (12) the kurtosis is then deduced:

$\begin{matrix}{{\kappa\left( {{\hat{a}}_{\ell}(f)} \right)} = {{\kappa\left( \overset{\sim}{a} \right)}\frac{\left( {\sum\limits_{n = 0}^{N - 1}{\sum\limits_{k = k_{0}}^{k_{K + 1}}{{\Lambda_{k,n}(f)}}^{4}}} \right)}{\left( {\sum\limits_{n = 0}^{N - 1}{\sum\limits_{k = k_{0}}^{k_{K + 1}}{{\Lambda_{k,n}(f)}}^{2}}} \right)^{2}}}} & (13)\end{matrix}$

Since κ(ã)=κ(a), it can now be checked whether the property (4) issatisfied:

$\begin{matrix}{{\kappa\left( {{\hat{a}}_{l}\left( {f + {\Delta\; f}} \right)} \right)} = {{\kappa(a)}\frac{\left( {\sum\limits_{n = 0}^{N - 1}{\sum\limits_{k = k_{0}}^{k_{K + 1}}{{\Lambda_{k,n}\left( {f + {\Delta\; f}} \right)}}^{4}}} \right)}{\left( {\sum\limits_{n = 0}^{N - 1}{\sum\limits_{k = k_{0}}^{k_{K + 1}}{{\Lambda_{k,n}\left( {f + {\Delta\; f}} \right)}}^{2}}} \right)^{2}}}} & (14)\end{matrix}$That is, given the expression (8) of Λ_(k,n)(f):

$\begin{matrix}{{\kappa\left( {{\hat{a}}_{l}\left( {f + {\Delta\; f}} \right)} \right)} = {{\kappa(a)}\frac{\left( {\sum\limits_{n = {- 1}}^{N - 2}{\sum\limits_{k = k_{0}}^{k_{K + 1}}{{\Lambda_{k,n}(f)}}^{4}}} \right)}{\left( {\sum\limits_{n = {- 1}}^{N - 2}{\sum\limits_{k = k_{0}}^{k_{K + 1}}{{\Lambda_{k,n}(f)}}^{2}}} \right)^{2}}}} & (15)\end{matrix}$and finally:

$\begin{matrix}{{{\kappa\left( {{\hat{a}}_{l}\left( {f + {\Delta\; f}} \right)} \right)} = {{\kappa(a)}\frac{\left( {\sum\limits_{k = k_{0}}^{k_{K + 1}}{\sum\limits_{n = 0}^{N - 1}{{{\Lambda_{k,n}(f)}}^{4}\left( {1 + ɛ_{k}^{(4)}} \right)}}} \right)}{\left( {\sum\limits_{k = k_{0}}^{k_{K + 1}}{\sum\limits_{n = 0}^{N - 1}{{{\Lambda_{k,n}(f)}}^{2}\left( {1 + ɛ_{k}^{(2)}} \right)}}} \right)^{2}}}}{with}{{ɛ_{k}^{(i)} = \frac{{{\Lambda_{k,{- 1}}(f)}}^{i} - {{\Lambda_{k,{N - 1}}(f)}}^{i}}{\sum\limits_{n = 0}^{N - 1}{{\Lambda_{k,n}(f)}}^{i}}},{i = 2},4.}} & (16)\end{matrix}$

It will be understood that the ε_(k) ⁽¹⁾s are bound terms and when Ntends towards infinity, the property κ(â_(l)(f+Δf))=κ(â_(l)(f)) isverified.

FIG. 3 schematically represents the flow chart of the OFDMidentification method according to one embodiment of the invention.

It is assumed that the sampling frequency of the signal received inbaseband is selected so as to be much higher than the commonly used chipfrequencies 1/T_(c).

In step 310, the width P of the sampling windows is chosen, designatedin numbers of samples, in other words the time width PT_(e) of thecutting off blocks of the signal received. In the case where one wantsto check the presence of a particular OFDM signal, having a given symbolduration T_(s), one will advantageously chose:

$\begin{matrix}{P = \left\lfloor {\lambda\frac{T_{s}}{T_{e}}} \right\rfloor} & (18)\end{matrix}$where λ is positive real number, preferably higher than 1, andadvantageously between 1.2 and 1.7, for example λ=1.5 as will be seenlater on. However, generally, one can proceed in a fully blind manner,that is without knowing beforehand the possible value(s) of T_(s), inwhich case a high enough value of P is chosen.

In step 320, the received signal is sampled and is cut off into blocksof P consecutive samples. It is assumed that the number of blocks thusacquired is L.

In step 330, for a plurality of frequencies f=f₀, . . . , f_(J), forexample for frequencies f_(j)=jδf, j=0, . . . , J−1 where δf is afrequency sample pitch, chosen low enough with respect to the subcarrierspacing of usual OFDM signals, the symbols carried by this frequency areassessed by Fourier Transform, typically using a FFT, that is accordingto (3):

${{\hat{a}}_{l}(f)} = {\sum\limits_{p = {l\; P}}^{{{({l + 1})}P} - 1}{{s_{a}\left( {pT}_{e} \right)}{\mathbb{e}}^{{2{\mathbb{i}}\;\pi\;{pT}_{e}f}\;}}}$where s_(a)(t) is the signal received in baseband. For each frequency f,a sequence of symbols â_(l)(f), l=1, . . . , L is thus obtained.

In step 340, for each frequency f, the kurtosis of symbols assessed atthis frequency is calculated, by means of the following expression,defined within a constant as:

$\begin{matrix}{{\kappa(f)} = \frac{\sum\limits_{l = 0}^{L - 1}{{{\hat{a}}_{l}(f)}}^{4}}{\left( {\sum\limits_{l = 0}^{L - 1}{{{\hat{a}}_{l}(f)}}^{2}} \right)^{2}}} & (19)\end{matrix}$

As has been seen herein above, if an OFDM signal is present in thereceived signal with the subcarrier spacing of Δf, the kurtosis κ(f) isa quasi-periodic function having a quasi-period Δf.

In step 350, it is determined whether the kurtosis κ(f) actually hassuch a periodicity. For example, the inverse Fourier Transform of thekurtosis κ(f) of which the continuous component thereof has beensubtracted therefrom, is calculated, that is:H _(P) =TF ⁻¹(κ(f)− κ)  (20)and it is determined whether there is a peak in the spectrum thusobtained.

Those skilled in the art will understand that further methods fordetermining the kurtosis periodicity can be used, in particular from themaximum of the autocorrelation function of κ(f).

The period or quasi-period thus determined is quoted Δ_(P)f and theheight of the spectrum peak or of the maximum of the autocorrelationfunction is quoted α_(P).

A detection threshold could be used to search the periodicity, forexample to determine whether the spectrum peak or the maximum of theautocorrelation function of the kurtosis is indeed significant. Othercriteria for detecting the periodicity of the kurtosis could becontemplated and this without departing from the scope of the presentinvention.

If no OFDM signal is present, the spectrum H_(P) does not have asignificant peak and no periodicity is detected in the kurtosis of theassessed symbols.

According to a first alternative, the identification method stops atthis step. This will be in particular the case when it is merely desiredto check whether an OFDM signal having a given symbol duration T_(s) ispresent in the received signal.

According to a second alternative, advantageously used in a fully blindmode, a range of sampling window width is scanned from a minimum valueP_(min) to a maximum value P_(max).

For this, it is tested in step 360 whether P=_(max). If no, the width ofthe sampling window is incremented in 365, P=P+δP, where δP is aninteger equal to or higher than 1, and then the process goes back tostep 320. On the other hand, if yes, the identification method proceedsto 370.

In 370, the width of the block P_(opt) having provided the highestspectrum peak height or the maximum of the autocorrelation value isdetermined, that is

$P_{opt} = {\arg\;{\max\limits_{P}{\left( \alpha_{P} \right).}}}$The period or quasi-period is then given by Δ_(P opt) f.

As above, a detection threshold α_(T) can be used to conclude that anOFDM signal is present or not. For example, the highest spectrum peakheight value, that is α_(P opt), can be compared with this thresholdvalue and it will be concluded that an OFDM signal is present only ifα_(P opt)>α_(T).

Whatever the alternative used, the identification method provides the Δfperiod and/or the corresponding useful duration of OFDM symbols.

FIG. 4A represents the variation of the kurtosis κ(f) of a group ofsymbols assessed for each frequency f, in the case of an OFDM signalpropagated in an additive white Gaussian noise (AWGN) channel. Therewill be noticed the periodic appearance of the function κ(f) symptomaticof the presence of an OFDM signal.

FIG. 4B represents the kurtosis spectrum of FIG. 4A. The normalizedvariable Δf/Δf₀ is shown in abscissa where Δf₀ is the subcarrier spacingof the OFDM signal actually present in the received signal. It will benoted the presence of a distinct peak in Δf/Δf₀=1, which means on theone hand that an OFDM signal has well been identified in the receivedsignal and, on the other hand, that its subcarrier spacing/usefulduration of the OFDM symbol has been correctly assessed.

FIG. 5A represents the variation of the kurtosis κ(f) of a group ofsymbols assessed for each frequency f, in the case of an OFDM signalbeing propagated in a multi-path channel. It is noticed that the curveappearance is substantially more disturbed than that of FIG. 4A but thatit however exhibits periodic variations.

FIG. 5B represents the kurtosis spectrum of FIG. 5A. Once again, thenormalized variable Δf/Δf₀ is shown in abscissa. The presence of a peakin Δf/Δf₀=1 is observed with the same conclusions as above, whichclearly shows the robustness of the identification method.

FIG. 6 represents the spectrum peak height α_(P) as a function of theratio

$\frac{{PT}_{e}}{T_{s}}$of the cutting off blocks width and the OFDM symbol duration, T_(s).

It is observed in this figure that the most pronounced spectrum peak isobtained when

${\frac{{PT}_{e}}{T_{s}} \approx 1},5,$in other words

${P_{opt} \approx 1},{5{\frac{T_{s}}{T_{e}}.}}$Thus, when one has to determine whether an OFDM signal having a symbolduration T_(s) is present or not, one will advantageously choseP=P_(opt).

The invention claimed is:
 1. An OFDM signal identification method foridentifying an OFDM signal in a received signal, implemented by aterminal, comprising: sampling and cutting off the received signal intoblocks of P samples, P being an integer; subjecting each block of Psamples to a Fourier transform to obtain a plurality of symbols at eachof a plurality of frequencies; calculating, for each frequency of theplurality of frequencies, kurtosis of a set of symbols obtained for acurrent frequency of the plurality of frequencies; determining whetherthe kurtosis calculated over the plurality of frequencies has aperiodicity in a frequency domain; and identifying presence of an OFDMsignal in the received signal when the kurtosis has such a periodicity.2. The identification method according to claim 1, wherein thedetermining of whether the kurtosis has a periodicity in the frequencydomain further comprises calculating a spectrum of the kurtosis anddetermining whether the kurtosis has a spectrum peak for a value havinga period.
 3. The identification method according to claim 2, wherein thedetermining of whether the kurtosis has a periodicity in the frequencydomain further comprises subtracting an average value of the kurtosisfrom the kurtosis to produce a result which represents the kurtosisbeing centered and calculating an inverse Fourier Transform of thekurtosis centered, comparing a spectrum height of the kurtosis centeredin the frequency domain and a predetermined threshold and detecting aspectrum peak when the spectrum height of the kurtosis centered in thefrequency domain is higher than the predetermined threshold.
 4. Theidentification method according to claim 1, the OFDM signal having agiven OFDM symbol time duration T_(s), and selecting the P samples perblock such that ${P_{opt} \approx 1},{5\frac{T_{s}}{T_{e}}},$ whereinT_(e) is the sampling period, P_(opt) is an integer.
 5. Theidentification method according to claim 1, wherein the cutting off ofthe received signal sampled into blocks of P samples is repeated for aplurality of different integer values of P, calculating kurtosis spectraby repeating the calculating of the kurtosis for the plurality of valuesof different integer values of P, determining a spectrum peak height ineach spectrum of the kurtosis spectra, and the determining of whetherthe kurtosis has a periodicity in the frequency domain further comprisesselecting a value P=P_(opt) corresponding to a highest peak height ofthe kurtosis spectra, wherein P_(opt) is an integer.
 6. Theidentification method according to claim 5, wherein, for the selectedP_(opt), value, the method includes determining whether the spectrumpeak has a height higher than a predetermined threshold value.
 7. Themethod according to claim 5, further comprising determining subcarrierspacing of the OFDM signal based on a position the spectrum peak heightin a kurtosis spectrum of the kurtosis spectra determined correspondingto the selected P_(opt), value.
 8. The identification method accordingto claim 5, further comprising determining a useful duration of an OFDMsymbol in the OFDM signal based on a position of the spectrum peakheight in a kurtosis spectrum of the kurtosis spectra determinedcorresponding to the selected P_(opt) value.